An electret condenser microphone has a wide variety of applications owing to its structural simplicity, compactness and reasonable fabrication cost. The electret condenser microphone has recently been used in a cellular phone and other portable audio devices. There is thus an increasing need to further reduce the size of and enhance the performance and reliability of the electret condenser microphone. To meet this need, Japanese patent application publication No. 11-088992 proposes a condenser microphone wherein a semiconductor chip, a conductive layer or a back electrode, an electret, a spacer and a movable diaphragm are stacked in that order.
Referring specifically to FIG. 5, there is shown, in section, a condenser microphone generally designated as at 30. The condenser microphone 30 includes a semiconductor chip 31 with a FET as an impedance converter and an amplifier. A conductive layer 32 is vacuum deposited on the semiconductor chip 31. A dielectric layer or electret 33 is formed on the conductive layer 32. A spacer 34 is printed on the outer peripheral edge of the electret 33. A movable diaphragm 35 is attached to the spacer 34 and cooperates with the electret 33 to form a working gap or air chamber 36.
The semiconductor chip 31, the conductive layer 32, the electret 33, the spacer 34 and the movable diaphragm 35 collectively form a microphone unit 37. A housing 38 is made of ceramic and encloses the microphone unit 37. The housing 38 has a front end wall, a rear end wall and a peripheral wall extending between the front and rear end walls. A plurality of sound inlet ports 38a are formed in the front end wall of the housing 38. A fabric or cloth 39 is attached to the front end wall of the housing. The semiconductor chip 31 includes two terminals or leads 31a, 31b. The two leads 31a, 31b extend through the rear end wall of the housing 38 and are soldered thereto.
As described above, the semiconductor chip 31 and all the other main components are integrated into a small unit. This arrangement enables the condenser microphone 30 to be economically manufactured on a mass production basis.
Japanese patent application publication No. 2003-230195 discloses a condenser microphone wherein a housing serves as an electric shield to inhibit entry of electric noise into the housing within which a microphone unit is contained. Referring specifically to FIG. 6, there is shown, in section, a condenser microphone 40 which includes a metallic housing 41. The housing 41 has a front end wall 43 and a cylindrical side wall 44 extending from the front end wall 43. A plurality of sound inlet ports 42 extend through the front end wall 43 of the housing 41. The front end wall 43 of the housing 41 acts as a fixed electrode. The housing 41 is formed on its inner surface with an electret 45. Disposed within the housing 41 are an annular electrically insulative spacer 46, an electrically conductive movable diaphragm 48 supported on a support ring 47 and acting as a movable electrode, and a cylindrical conductive ring 49.
The housing 41 has an open rear end in which a printed circuit board 51 is arranged. A plurality of electronic components 50 such as a FET are surface mounted onto the inner surface of the printed circuit board 51. The printed circuit board 51 is provided on its outer periphery with a conductive layer or ground section 51a. The lower end of the side wall 44 is radially inwardly curved to form a bent end 44a. The bent end 44a makes contact with the ground section 51a to provide an electrical connection between the housing 41 and the ground section 51a. This arrangement electrically isolates the interior of the housing 41 from the outside of the housing 41. A filter 52 is attached to the outer surface of the front end wall 43 of the housing 41 and is made of a non-woven fabric, a cloth and other materials. The front end wall 43 of the housing 41 and the movable diaphragm 48 collectively constitute a condenser. With this arrangement, a change in electrical capacitance between the front end wall 43 of the housing 41 and the movable diaphragm 48 occurs when the diaphragm 48 is vibrated or deflected in response to an incident sound pressure wave through the sound inlet ports 42. Such a capacitive change is converted to an impedance by means of the electronic components 50 and then output as an electrical signal from a terminal 51b. The terminal 51b is formed on the outer surface of the printed circuit board 51.
Again, the ground section 51a and the bent end 44a of the metallic housing 41 are connected to electrically isolate the interior of the microphone unit. The condenser microphone 40 is thus capable of preventing entry of electric noise into the interior of the microphone and providing a relatively low signal to noise ratio.
A problem with the condenser microphone 30 shown in Japanese patent application publication No. 11-088992 is that water may enter the interior of the housing 38 if clearances are left between the side wall and the rear end wall of the housing and between the through holes in the rear end wall of the housing and the corresponding leads 31a, 31b, as shown by the imaginary arrows in FIG. 5. The water, when entered, can oxidize the surface of the movable diaphragm 35. Obviously, such oxidization adversely affects the sensitivity and the frequency characteristics of the movable diaphragm 35. This problem becomes more serious particularly in case that the movable diaphragm is formed with a plurality of perforations (not shown). In such a case, the water may even flow over the rear side of the movable diaphragm 35. This further deteriorates the sensitivity and the frequency characteristics of the movable diaphragm. The clearances also create another problem. Sound pressure waves normally move into the housing 38 through the sound inlet ports 38a and cause the movable diaphragm 35 to vibrate or deflect. If the clearances are formed in the housing 38 as shown in FIG. 5, the sound pressure waves can enter the interior of the housing 38 through the clearances. This alters the directionality of the microphone and adversely affects the frequency characteristics of the microphone.
There is also a drawback to the condenser microphone 40 shown in Japanese patent application publication No. 2003-230195. The electrical connection between the bent end 44a of the housing 41 and the ground section 51a of the printed circuit board 51 may be damaged if dust or water droplets are attached thereto. If this occurs, the housing 41 and the ground section 51a of the printed circuit board 51 will have a resultant high electrical resistance, and the housing 41 will no longer act as an electric shield. As a consequence, electric noise (or burst noise) is free to enter the housing and significantly lowers the performance of the microphone.
It is, therefore, an object of the present invention to overcome the foregoing drawbacks and provides a reliable condenser microphone which can prevent entry of water into the interior of the microphone and also, entry of sound pressure waves into the housing of the microphone through a portion of the microphone other than predetermined sound inlet ports, and which can maintain the sensitivity, the frequency characteristics and the directionality of the microphone. It is another object of the present invention to provide a high performance condenser microphone which can exhibit a high level of electrical shielding effect.